The invention relates to a photoresist compound or composition, as well as a method for structuring a photoresist layer.
In semiconductor technology, photolithographic methods for producing integrated circuits on a semiconductor substrate play a central role. In these methods, photoresist layers are typically applied onto the surface of a substrate that is to be structured, and are subsequently structurally exposed with radiation from a suitable wavelength range. Here, the structural exposure takes place using a lithography mask, through which the structure that is to be transferred into the substrate is predetermined. The exposed regions of the photoresist layer are chemically modified by the exposure, thus modifying their polarity. In this way, the exposed and unexposed regions of the photoresist have different solubility's in relation to a corresponding developer. This fact is exploited in the subsequent developing step for the selective removal of the exposed or unexposed regions. The regions of the photoresist layer remaining on the substrate are used in the following structuring step as a mask, which protects the substrate layer located underneath it from a wearing away of material or modification of the material. Such a structuring step can be for example a plasma etching, a wet-chemical etching, or an ion implantation.
The photoresists used for the structuring of semiconductor components must meet a multiplicity of demands. On the one hand, they must enable the transfer of structures that are increasingly becoming smaller into the layer to be structured. On the other hand, the photoresist layers must ensure a sufficient edge coverage of the structures already produced on the substrate in preceding process steps. While the precise transfer of structures that are as small as possible generally requires the use of photoresist layers that are as thin as possible, a sufficient edge coverage is generally achieved using relatively thick photoresist layers. In order to meet these opposed requirements, two-layer techniques, or multilayer techniques, are often used. For example, a two-layer photoresist is used in which, on a lower, relatively thick, planarizing photoresist layer (bottom resist), a second, relatively thin photoresist layer (top resist) is applied. The structure produced according to conventional methods in the top resist is subsequently transferred into the bottom resist in an anisotropic etching method, e.g. using an oxygen/RIE plasma method, the developed top resist structure being used as a mask. Through this two-layer technique, the dimensions of the structures that can be imaged can be significantly reduced, i.e., the resolution can be significantly improved.
Chemically strengthened resists (chemical amplification resists; CAR) have proven particularly effective as photoresists both for one-layer resists, which can be wet-developed, and for two-layer resist systems, which can be partly or entirely dry-developed. In these photoresists, photoacid generators can be used as photosensitive compounds. An overview of the subject is given by H. Ito in Solid State Technology, July 1996, p. 164 ff. In a selected group of these systems, the solubility modification is achieved using the principle of acid-catalyzed separation or decomposition. This principle can be used both in resists that operate positively and those that operate negatively. In the case of a positive resist, in a heating step a polar molecular group, for example a carboxylic acid, is formed from an unpolar chemical group, for example a carboxylic acid tert.-butyl ester group, in the presence of a photolytically produced acid. Further examples of unpolar “blocked” groups that can be converted into corresponding polar groups through acid-catalyzed reactions include the tert.-butoxycarbonyloxy (t-BOC) or acetal groups. Through the conversion of the unpolar groups into the corresponding polar groups, the resist undergoes a change of polarity in the previously irradiated regions, and thus becomes soluble in relation to a polar, aqueous-alkaline or diluted alkaline developer. In this way, the exposed regions of the photoresist can be removed selectively by the developer.
The resolution, i.e., the smallest imageable structure (critical dimension: CD) that can be achieved in the photolithographic production of resist structures, is determined by the photolithographic properties of the photoresist material, the wavelength of the radiation used for the exposure, and the numerical aperture of the imaging optics. This dependence essentially allows an improvement of the resolution to be made only by enlarging the numerical aperture or by reducing the wavelength of the radiation used for the exposure. However, the use of particularly shortwave radiation requires the development of special photoresists that combine suitable absorption properties with the other required properties, such as for example etching resistance and sufficient ability to be developed. Due to this, the number of suitable photoresists is sharply limited.
A completely different approach, through which an improved resolution can be achieved independent of the wavelength of the radiation used for the exposure, is what is known as the two-layer CARL method, as described for example in U.S. Pat. No. 5,234,793 whose disclosure is incorporated by reference thereto and which claims priority from the same German application as European patent EP 395 917. In this method, the developed top resist structure of a two-layer photoresist is modified through an additional chemical expansion step before the etching of the bottom resist. Here, the developed top resist structure is exposed to an expansion agent containing an expansion or bulging component. The expansion component forms a chemical linkage with the functional groups of the photoresist, resulting in a volume-increasing building of the expansion component into the top resist. This building in achieves a defined broadening of the top resist structure through lateral layer thickness growth, as well as a defined increase in height through vertical layer thickness growth. Here the lateral and vertical layer thickness growths can differ markedly.
However, a problem in this method consists in the achieving of a constant layer thickness growth within all regions of the top resist structure. Frequently, the vertical layer thickness growth on the surface regions of the photoresist structure is uniform, whereas, in contrast, a non-uniform lateral layer thickness growth can be observed on the edge regions of the photoresist structure.
This problem is connected with the absorption of radiation by the photoresist material during the structural exposure. In the chemical expansion of chemically reinforced photoresists, the layer thickness growth rate increases as the radiation dosage of the structural exposure increases. This can be explained in that, as the exposure dosages increase, there is an increased production of polar groups in the photoresist by the photoacid generator, and these polar groups influence the layer thickness growth rate. Because most of the functional groups that can react with the expansion component are likewise polar hydrophilic groups, as a rule the growth process is based on an equilibrium between the capacity of the exposed photoresist to be developed and its capacity to be expanded. This equilibrium can be influenced in directed fashion by the polar character of the base polymer in the photoresist. The expansion agent can thus likewise be used as a developer.
During the structuring exposure and the subsequent development of a photoresist film, defined resist profiles are produced. According to the orientation of the surface of the individual regions of the resist profile, it is possible to distinguish between the profile upper edges, whose surface runs essentially parallel to the surface situated under the photoresist, and the profile edges, whose surface runs essentially perpendicular to the surface situated under the photoresist. The exposure dosage within the top resist film decreases continuously as the layer thickness increases, due to the intensity losses caused by radiation absorption. As a consequence, in chemically reinforced photoresists the number of polar hydrophilic groups that are released by the photolytically produced acid decreases in the same manner within the layer. This has the consequence that, as the depth increases, the profile edges have a changed polarity, and thus a changed hydrophilic character. This polarity gradient within the profile edge regions results in a modified lateral layer thickness growth as the profile depth increases. This has a decisive influence on the lateral wall profile produced with the expansion step. A uniform layer thickness growth on the profile edges is of decisive importance for the quality of the photoresist mask, and thus for the resolution of a structuring step, such as for example a substrate etching. Masks having almost perpendicular or vertical profile edges, which enable a structuring of the top resist that is as anisotropic as possible, are preferred for subsequent process steps. This requirement is of particular importance due to the increasing scale of semiconductor components.